The Ultimate Guide to the 3 Phase Induction Motor: Construction and Working Principle

Three-phase induction motors do a lot of the hard work in our world today. You can find these strong induction motors in many places. They are in very big factory machines. They are also in the pumps that give you water. This article is a simple guide that will help you understand them. We will explain how a 3 phase induction motor is built and how it works, using easy steps. By the time you finish reading, you will know how this great electric motor works. You will also know why it is used so much.

What Really Makes a 3-Phase Induction Motor Go?

A 3 phase induction motor is a kind of AC motor. In this motor, power is given to the rotor part using electromagnetic induction. This is the same main idea that a transformer uses. That is why people sometimes call these induction motors “rotating transformers.” The important thing to know is that there is no wire that connects directly to the part that moves, which is the rotor.

This type of motor is famous because it is built in a simple and strong way. It does not have parts called brushes, which some other motors have. Because of this, it does not need much care or many repairs. These induction motors also start by themselves. This is a big plus when you compare it to a single-phase induction motor. Because they are so simple and you can count on them, 3-phase induction motors are the most common type of ac motor you will find in factories. The main job of this phase induction motor is to change three-phase electricity into motion.

What Are the Two Big Parts Inside Induction Motors?

Every phase induction motor has two main parts. If you understand these parts, you can start to understand how the whole motor works. The induction motor consists of two important pieces:

1. The Stator: This is the part of the motor that stays still. It does not move at all. Its job is to make a magnetic field when you connect it to a three-phase AC power source. The stator is a steel frame that holds coils of wire. This group of coils is called a winding.
2. The Rotor: This is the part of the motor that spins. It sits inside the stator. The rotor has its own set of metal bars or wires. When the stator’s magnetic field moves past the bars on the rotor, a current is made inside them. This current makes its own magnetic field. This is what pushes the rotor and makes it spin. This is the magic behind induction motors.

There is a very small space between the stator and rotor. This space is the air gap. The air gap is made as small as possible to help the motor work better. How these two main parts work together is what makes all induction motors turn a shaft and get work done.

How is the Stator of Three Phase Induction Motors Put Together?

The stator of three phase induction motors is built with a lot of care. It is shaped like a hollow tube and is made of thin sheets of high-quality steel. These thin steel sheets are pressed together in a stack. Using thin sheets helps to lower the amount of energy that gets wasted. Inside the stator, there are cuts called slots. The stator winding is put into these slots. The winding is made from copper wire that has a special coating to keep the electricity from leaking.

This winding is not just one big loop of wire. It is really three different sets of windings. Each winding is for one phase of the three-phase ac supply. The windings are put into the slots in a very specific pattern. They are set 120 degrees away from each other. When you hook up a 3-phase ac power source to this three-phase winding, it makes a special kind of magnetic field. This field is the key to how all induction motors work. The stator is wound for a definite number of poles. The number of poles helps to set the motor’s speed. The motor consists of this very important part.

What is a Squirrel Cage Rotor, and Why Do So Many Motors Use It?

The kind of rotor you see most often is the squirrel cage rotor. It gets its name because of its shape. It looks a bit like a running wheel for a small animal. This rotor is made from a core of stacked, thin steel sheets. It has slots on its outside surface. This rotor does not have a wire winding. Instead, it uses thick bars of copper or aluminum. These bars are put into the rotor slots.

All of the bars are joined together at each end. They are connected by thick metal rings called end rings. This makes a full electrical path. The copper bar and end rings look like a cage, which is how it got the name squirrel cage. This design is very tough and not complicated. It has no brushes or slip ring parts that can break or wear down. The squirrel cage induction motor has a very robust construction. This makes it a machine you can count on, and it is not expensive to make. This is the reason that more than 9 out of 10 induction motors use this kind of rotor. The rotor of a three-phase induction motor is almost always this squirrel cage type.

Is There a Different Kind of Rotor? Let’s Talk About the Wound Rotor.

Yes, there is another kind of rotor. People call it a wound rotor or a slip ring motor. This type of rotor is not like the squirrel cage version. A wound rotor has a full 3 phase winding, much like the winding in the stator. It does not use solid bars. The winding is made of coated copper wire and is placed into the slots on the rotor. This is why we call it a phase wound rotor.

The ends of this rotor winding are not connected to each other like in a squirrel cage. Instead, they are wired to three special metal rings that are on the shaft. These are called slip rings. Small carbon blocks, called brushes, press against these slip rings. This lets you connect outside parts, like resistors in the rotor circuit. By adding resistance, you get more control over how the motor works. For example, it gives the motor a much stronger starting torque than a squirrel cage motor. A slip ring induction motor is used for jobs that need a lot of power to get started. This wound motor is more difficult to build and costs more money.

How Does the Main Idea of Electromagnetic Induction Make These Induction Motors Run?

The working principle for all induction motors comes from a science rule called Electromagnetic Induction. This rule says that if you move a wire (a conductor) through a magnetic field, a voltage (emf) will be made, or induced, in the wire. The same thing happens if the magnetic field is moving and the wire is still. What matters is that one is moving compared to the other. This is called relative motion.

In a 3 phase induction motor, the stator winding makes a magnetic field that spins around. This spinning field moves past the metal bars of the rotor. The rotor bars are all connected, making a full circle for electricity. The induced emf makes a current flow through them. This current flows in the rotor bars. Now you have a conductor with current in it (the rotor) sitting inside a magnetic field (from the stator). This makes a turning force, or torque, on the rotor. This torque is the force that makes the rotor spin. The power gets from the stator to the rotor just by using electromagnetic induction from the stator.

What is a Rotating Magnetic Field, and Why is It a Big Deal?

A rotating magnetic field is the secret that makes three-phase induction motors work so well. When you connect a good three-phase power source to the stator winding, something cool happens. Each of the three windings makes its own magnetic field. This field gets stronger and weaker as the AC power changes. But because the windings are spaced 120 degrees from each other, their magnetic fields add up to create one big magnetic field.

This new, combined magnetic field does not stay in one place. It spins around the stator at a steady speed. We call this speed the synchronous speed. The synchronous speed is set by two things: how fast the AC power source cycles (its frequency) and the number of poles the stator was made with. It is this rotating magnetic field that moves past the rotor’s conductors and starts everything. If it did not have this field, the phase induction motor would not be able to start on its own. The magnetic field of the stator has to spin.

What is ‘Slip’ When We Talk About an Induction Motor?

The rotor of an induction motor will always spin a little bit slower than the stator’s rotating magnetic field. The difference in speed between the synchronous speed of the field and the real speed of the rotor is what we call slip. We usually talk about slip as a percentage. If the rotor could spin at the exact same speed as the magnetic field (at synchronous speed), then the field would not be moving past the rotor.

If there is no motion between them, the magnetic field would not cut through the rotor conductors. This means no emf would be made, and no current would flow in the rotor. If there is no current, there is no torque. Without torque, the rotor would slow down. So, in order for the motor runs, the rotor must spin slower than the magnetic field. This is the only way to make torque. This is also why an induction motor is sometimes called an asynchronous motor (meaning not in sync). The amount of slip gets bigger when you put more work on the motor. A normal induction motor may have a slip of 3% to 5% when it’s working at its hardest.

What is the Right Way to Connect a 3-Phase Induction Motor?

Most 3-phase induction motors have a box with six connection points. These points are the ends of the three windings inside the stator. There are two main ways to connect them: Star (also called Wye) and Delta. The way you choose to connect them changes how the motor will run.

• Star Connection (Y): For this connection, you join one end of all three windings together at one point. Then, you connect the 3-phase power to the other three ends. People often use this connection to start the motor. It lowers the voltage for each winding. This makes the starting current lower, which can be better for the building’s power system. But, it also gives you less starting torque.
• Delta Connection (Δ): For this connection, you connect the windings end-to-end. This makes them form a triangle shape. Then, you connect the 3-phase power to the three points where the windings meet. In a Delta connection, every winding gets the full voltage from the power line. This gives you more starting torque, but it also pulls a lot more starting current. Many bigger induction motors use a special “Star-Delta Starter.” This starter begins in the Star connection and then changes to Delta after the motor gets up to speed.

Are There Any Downsides? The Disadvantages of 3 Phase Induction Motors.

Even though 3-phase induction motors are great machines, they are not perfect. They have a few weak points you should know about. Here are the main disadvantages of 3 phase induction.

• Bad Power Factor With a Light Load: The power factor of an induction motor is not very good when it is running without much work to do. A low power factor is not good for the power company.
• Hard to Control Speed: It is not easy to change the speed of a regular squirrel cage induction motor. Its speed is very closely linked to the frequency of the power it gets. You need special, costly gear (like a Variable Frequency Drive) for good speed control of 3-phase induction.
• Low Starting Power (for Squirrel Cage type): When you compare it to other kinds of motors (like DC motors or slip ring motors), the normal squirrel cage induction motor does not have a very strong starting torque. This means it is not a good choice for jobs that have to start with a very heavy load.
• High Starting Current: When a phase induction motor first starts up, it can pull in a huge amount of electricity. This is often 5 to 7 times more than its normal running current. This can make the lights dim for a moment as the voltage in the power lines drops.

Even with these problems, the good things about induction motors, like the fact that they are very reliable and do not cost a lot, make them the top choice for many, many jobs.

Key Things to Remember

• Induction motors use the idea of electromagnetic induction. There are no wires that connect directly to the spinning rotor.
• The two main parts are the stator, which stays still, and the rotor, which spins.
• The stator makes a rotating magnetic field when it is connected to three-phase power.
• The most common kind of rotor is the squirrel cage rotor. It is used so much because it is simple, strong, and does not cost much.
• The wound rotor, also called a slip ring motor, gives you very strong starting torque and more control. But it is more complicated and costs more.
• Slip is the speed difference between the stator’s field and the rotor. You need slip to make torque.
• You can connect induction motors in a Star (Y) or Delta (Δ) shape. This changes the starting current and torque.